This series has a slightly complicated demographic tag situation. It was originally published in Comic Blade Masamune, a shounen magazine. This was replaced by Comic Avarus, a shoujo magazine, right in the middle of various ongoing series. They decided to just run the rest of some of those in there anyway. So, shoujo but not shoujo?
Those "BL-undertones" in the anime were likely meant to appeal to the magazine's new target demographic I guess. Kind of a dick move though. A lot of shounen readers would see that anime come out and never try the manga, shoujo readers would see it and check out the manga only to find it relatively lacking in that regard because it was still pretty much a shounen series despite the magazine.